Alex Salmond has tried to bolster the Scottish National party's claims to be local champions for voters by asking all his MPs to sign pledges to be "open and accountable" to their constituents.

The SNP leader claimed the new "community commitment" would require each nationalist MP to guarantee "accessibility, accountability and openness" as a form of contract between each nationalist candidate and local voters, stressing that the MPs would be champions for their immediate areas.

A senior party official said Salmond and his Welsh nationalist counterparts in Plaid Cymru were trying to portray themselves as a form of "insurgency" or irregular force working outside the "political machine" represented by the main UK parties.

Over the past few days the first minister of Scotland has developed a new theme: that after the expenses scandal Westminster is now a discredited parliament. At the weekend, to mark his retirement from the Commons after 23 years as an MP, he released a video attacking Westminster's recent record.

Speaking in Edinburgh today , Salmond said: "The House of Commons is now held in widespread disrepute. The best remedy is to offer a different style of MP: local champions whose mission is to articulate community concerns."

With six MPs in the last parliament, the SNP leadership fears the party's message will be drowned out at UK level by the battle for control at Westminster between Labour and the Tories, so it is fighting to prove that even a small number of nationalist MPs can offer a viable and real alternative.

Salmond has set his party a target of 20 MPs but opinion polls suggest the party will struggle to win many more seats and could lose some, particularly Glasgow East. Plaid Cymru launches its manifesto tomorrow in an effort to win at least four seats at Westminster in the hope of working in coalition with the SNP.

With speculation about a hung parliament intensifying, the Liberal Democrats are emerging as the party that will have the greatest influence at Westminster after the election, a factor that threatens further to isolate the Welsh and Scottish nationalists.

Salmond repeated his attacks on Labour and Conservative economic policies, alleging that both parties would cut Scotland's budget over the next 15 years by at least £30bn in total as they moved to slash public spending.

"Neither Tory not Labour want the detail of their plans to be scrutinised in this campaign, but an analysis of the budget demonstrated that they are both planning a decade of despair in public services," he said.

Jim Murphy, the Scottish secretary, derided the SNP's claims. He said this was the third time Salmond had launched his party's campaign – a reference to an event last week and another last month, at which the SNP leader unveiled his party's "local champions" strategy.

Speaking as he launched the Scottish version of Labour's election manifesto – this is the first time the party has published parallel manifestos for Scotland and England – Murphy said SNP events "are launches that nobody notices because they are a party in this election without a purpose and without a point. It's a desperate attempt by them to be noticed."

Voting SNP would be a wasted vote since it was a small minority party, Murphy said. "Most people in Scotland know this: that a vote for the SNP will let the Tories in by the back door," he said.

Murphy's theme was echoed by Alistair Carmichael, the Lib Dem Scottish campaign manager, who is defending his seat in Orkney at the election. "Instead of launching their campaign for the third time, the SNP should be heading for the lifeboats," he said.

"Their campaign is holed below the waterline because they are irrelevant. No amount of rebranding or relaunching is going to change that."

However Murphy also focused on the Tories and their economic record under Margaret Thatcher's governments 20 years ago and its current policies.

Labour's Scottish campaign was launched at a new £70m college commissioned by the last Labour-Lib Dem coalition government of Scotland on the site of Ravenscraig steel works, where 25,000 people lost their jobs during Thatcher's prime ministership.

He denied the closure of Ravenscraig was an event of little relevance today. Then, the Tories left 25,000 former steel workers to "sink or swim", he said.

"It is right, fit, proper and positive to say that the Conservatives still don't understand Scotland. It's a contrast of ideas and a contrast of values."